We report a patient with Prader-Willi syndrome (PWS) and mosaicism for a de novo jumping translocation of distal chromosome 15q, resulting in partial trisomy for 15q24-qter. A maternal uniparental heterodisomy for chromosome 15 was present in all cells, defining the molecular basis for the PWS in this patient. The translocated distal 15q fragment was of paternal origin and was present as a jumping translocation, involving three different translocation partners, chromosomes 14q, 4q, and 16p. The recipient chromosomes appeared cytogenetically intact and interstitial telomere DNA sequences were present at the breakpoint junctions. This strongly suggests that the initial event leading to the translocation of distal 15q was a non-reciprocal translocation, with fusion between the 15q24 breakpoint and the telomeres of the recipient chromosomes. These observations are best explained by a partial zygotic trisomy rescue and comprise a previously undescribed mechanism leading to partial trisomy.

During a systematic survey of the mentally retarded, 3 related females were seen with a similar syndrome of shortness, unusual combination of craniofacial anomalies (trigonocephaly; bulging forehead; flat face; posteriorly angulated, lowset ears and microretrognathia), and genital hypoplasia in all 3 cases, and multiple pterygia in one. The facial changes were also noted in 2 grandmothers and may indicate autosomal dominant inheritance of this presently “private” MCA/MR syndrome.

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In this report a severely mentally retarded adult female with 9p trisomy/18pter monosomy is described. In addition to a 9p trisomy phenotype this patient presented with multiple cutaneous leiomyomata. The question is raised whether the concurrence of the chromosomal anomaly and the multiple skin tumors in this patient indicates another example of a specific chromosomal deletion (18pter) in a dominantly inherited multiple human tumor.